A three-dimensional NAND flash memory in which memory cells are three-dimensionally disposed realize a large integration and low cost. In order to improve read characteristics of a three-dimensional NAND flash memory, a positive feedback type three-dimensional NAND flash memory having a p-type semiconductor region at one end of a memory string has been proposed. The positive feedback type three-dimensional NAND flash memory has a sharp subthreshold slope since a transistor of a memory cell performs a thyristor operation. Accordingly, the read characteristics of the memory cell are improved.
In order to increase the capacity of the three-dimensional NAND flash memory, memory cells may be set into multi-levels to each store data of a plurality of bits. To set memory cells into multi-levels, threshold voltages of the memory cells (amount of electric charge accumulated in charge accumulation regions of the memory cells) should be controlled to have a plurality of distributions in a write operation. It is important to narrow the widths of the threshold voltage distributions of the memory cells in order to prevent/suppress erroneous reading in a read operation. Even in the positive feedback type three-dimensional NAND flash memory, when memory cells are set into multi-levels, it is desired to narrow the width of the threshold voltage distributions of the memory cells.